The proliferation and differentiation of hematopoietic cells is under the control of hematopoietic growth factors. A variety of cell types are capable of producing these growth factors including endothelial cells, fibroblasts, stromal cells, monocytes and lymphocytes. Growth factors stimulate both proliferation and/or differentiation of hematopoietic cells. There is substantial redundancy in the spectrum of activity of hematopoietic growth factors in that several individual factors may act on mature and immature cells of several lineages and there are several factors that have overlapping spectra of activity. One goal is to devise strategies to determine the in vivo role of particular factors in both hematopoiesis and lymphoid cell differentiation. We have devised an expression vector that allows an overproduction of antisense RNA sequences in T-lymphoid cells. Our in vitro data indicate that such antisense sequences will inhibit growth factor production. Transgenic animals expressing the antisense sequences in T-lymphocytes should be deficient in the production of a particular growth factor, providing an opportunity to determine, by analysis of the deficient phenotype, the role of that factor in hematopoiesis and lymphoid cell proliferation. The second series of experiments is directed at the role of particular early response gene products in the response to hematopoietic growth factors. The AP-1 family of proteins is composed of JUN and FOS species that form various homo- and heterodimers and act as transcriptional modulators of gene expression. We have found that the pattern of early response gene expression in two closely related cell lines is specific for the cell line and unrelated to the growth factor used to activate cell proliferation. A third series of experiments involved introduction of the M-CSF receptor in a primitive hematopoietic cell line to determine whether it would influence the pattern of differentiation. We found that the receptor stimulated with M-CSF induced a mitogenic response, but that the pattern of early response gene expression was identical to that invoked by IL-3 and that no change in the differentiation pattern of the cells was observed.